Skripal was found slumped unconscious on a bench in Salisbury on Sunday afternoon. The woman discovered next to him has been identified as Skripal’s 33-year-old daughter, Yulia. Both remain critically ill and in intensive care after exposure to an “unknown substance”.

Yulia Skripal. Photograph: AP

The Metropolitan police said that due to the “unusual circumstances” its counter-terrorism unit would now be heading the investigation. The police presence in Salisbury was stepped up on Tuesday, as specialist officers appeared around a police cordon.

Sources close to British intelligence said toxicology tests would be key in the days ahead. They cautioned that other factors or triggers may have been involved. Samples from the scene are being tested at the military research laboratory at Porton Down. Experts have yet to identify the substance.

Witnesses said that Yulia Skripal had “passed out”, adding that her father was “looking at the sky”. “They didn’t seem with it,” said Freya Church. “To be honest, I thought they were just drugged out as they were in a weird state. They were out of it.”

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Detectives were closely scrutinising CCTV footage. It shows a man and woman – possibly Skripal and his daughter – shortly before 4pm walking together normally past a fitness centre. Soon afterwards both collapsed.

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Russian spy mystery: police release CCTV footage – video

The restaurant Zizzi in the city centre and a nearby pub, the Mill, remain closed as a precaution, police said.

Firefighters spent around an hour on Tuesday at South Western Ambulance’s base at Amesbury. It is understood the incident is connected to events in Salisbury.

A worker at the Mill said Skripal and his daughter came into the pub on Sunday afternoon, enjoyed a drink and a laugh and left together. “All seemed completely normal,” he said.

Who was Alexander Litvinenko?

Alexander Litvinenko had been an officer with Russia's Federal Security Service (FSB), the successor to the KGB, but fled to Britain where he became a fierce critic of the Kremlin, and a British citizen.

Litvinenko fell ill and died in November 2006 after drinking a cup of tea laced with radioactive polonium, sparking a major international incident.

He met his killers in the Millennium hotel in Mayfair, central London. The pair were Andrei Lugovoi – a former KGB officer turned businessman, who is now a deputy in Russia’s state Duma – and Dmitry Kovtun, a childhood friend of Lugovoi’s from a Soviet military family. Vladimir Putin denied involvement and refused requests to extradite either of the killers from Moscow.

A public inquiry in 2015 and 2016 heard five months of evidence, including secret submissions from UK spy agencies. Its chairman, Sir Robert Owen, concluded that the FSB had murdered Litvinenko, assigning Lugovoi and Kovtun to carry out the mission.

Owen also ruled that Putin had “probably approved” the operation.

Photograph: Natasja Weitsz/Getty Images Europe

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The foreign secretary ignited a political row with Moscow by warning that if suspicions about Kremlin involvement were again confirmed England might pull out of this summer’s World Cup, to be hosted by Russia. “I think it will be very difficult to imagine that UK representation at that event could go ahead in the normal way,” he told MPs.

He added: “We don’t know exactly what has taken place in Salisbury, but if it’s as bad as it looks, it is another crime in the litany of crimes that we can lay at Russia’s door. It is clear that Russia, I’m afraid, is now in many respects a malign and disruptive force.”

Foreign office aides swiftly clarified that Johnson meant government officials, rather than England players. Russia’s foreign ministry dismissed his comments as “wild” and accused the UK of hysteria and unwarranted “Russophobia”. The embassy in London said “there was no truth” in the suggestion its special services were connected.

The home secretary, Amber Rudd, is due to chair a meeting of the government’s Cobra emergency committee on Wednesday morning in response to the incident.

If a toxin or nerve agent is confirmed then the government will face a major international crisis and – inevitably – an acrimonious war of words with Russia. Theresa May was briefed about the incident in Salisbury during a meeting on Tuesday of the national security council, her spokesman said.

Britain would not win a spy showdown with Putin | Richard Norton-Taylor

Skripal had been living quietly in a semi-detached house in Salisbury. His wife, Liudmila, died of cancer in 2012. Neighbours described him as friendly, sociable and living in full view. He shopped locally, drank in the Railway social club and gambled. Skripal sometimes admitted he had been a colonel in Russian intelligence, but told others he was a retired former planning officer.

The incident on Sunday triggered a full emergency response. Skripal was taken to hospital by ambulance, while his daughter was flown there by helicopter. According to her Facebook page, she moved to the UK in 2010 with her parents, working for a time at the Holiday Inn in Southampton. Skripal’s 43-year-old son appears to have died recently during a visit to St Petersburg.

The biggest question about Skripal’s mysterious poisoning is the timing. Skripal had spent several years in a Russian jail and had presumably been thoroughly debriefed. If the Russian security services had wanted him to have an “accident” during those years, it would have been very easy to organise.

On Monday Putin gave a speech to his old FSB service, congratulating officers on their success in detecting foreign espionage plots. He will be reelected as president later this month. Whether or not the Kremlin’s fingerprints are found, the Skripal case is likely to be seen as a warning to Russian operatives of the dangers of working with MI6 or the CIA.

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Shoppers returned to the centre of Salisbury on Tuesday. Public Health England said there didn’t appear to be any immediate risk to public health. One member of the emergency services remained in hospital, Wiltshire police said, with several others treated on Sunday now discharged.

The US State Department declined to comment on the case, referring the matter back to its counterparts in the UK.

Marina Litvinenko, the widow of Alexander Litvinenko, said no conclusions should be made until “the facts and evidence” are available. “I didn’t know Sergei but it looks very much a spy story,” she told the Guardian. “He has never been involved in any political activity, never criticised Putin.”